The preoperative selection of intraocular lenses for cataract treatment constitutes an important application. The most important length measurement for this purpose is the axial length of the eye from the front of the cornea to the retina. According to the prior art, said length is measured preferably with non-contact, optical interferometric methods which are known under the terms PCI (partial coherence interferometry) or OCT (optical coherence tomography). With these methods, structural transitions can be displayed as one-dimensional depth profiles (A-scans) or two-dimensional depth cross sections (B-scans), wherein specular reflections at the optical boundary layers and/or light which is scattered to the various media of the eye are detected.
It is important for both measurement methods that the measurement is taken along an axially oriented axis which corresponds to the visual axis. Otherwise, errors may occur during the selection of the IOL which result in significant defective vision of the patient after implantation of the IOL.
In order to ensure great accuracy of the measurement along the visual axis, the patient, according to the prior art, is provided with a fixation light from the optical measuring device during measurement, onto which the patient fixates the eye. Thus, the visual axis of the eye is aligned with the main measuring axis of the device (device axis) which also corresponds to the z-axis of the coordinate system of the measuring device. This can be found in the literature [1]. Once the device axis is aligned with the visual axis, cornea and retina are, in most cases, positioned sufficiently perpendicular to the main measuring axis, and so the measuring beams reflected from cornea and retina are easily detectable by the measuring device.
According to a first method described in the literature [2], the axis length is measured using partial coherence interferometry with the double-beam method. Two beams with different optical path lengths impinge on the eye and are specularly reflected on the front of the cornea and the retina, resulting in interference. The signals at different optical path lengths are indicative of the eye length. Since a usable signal is only generated when a specular reflection is present from both cornea and retina, this method is advantageous because cornea and retina are approximately perpendicular to the measuring beam, and thus perpendicular to the device axis, for generating a cornea/retina distance signal. Experimentally, it has been shown that under these measurement conditions, which lead to a usable distance signal, the device axis/measuring axis is, in good approximation, identical with the visual axis, and the distance measured along the device axis corresponds to the axial length which is determinative for calculating the IOL. This measurement method quasi-inherently ensures that in case of too great a deviation of the visual axis from the device axis, no incorrect measurement of the eye length is obtained and used for calculating the IOL.
However, it is disadvantageous that, for the duration of the measurement time, the patient must summon a minimum of cooperation for the fixation. If this is not the case, no or very few, and thus statistically fairly unreliable, measurements can be determined for the axial eye length.
It is further disadvantageous that measurements for B-scans or the measuring of the anterior chamber depth are difficult to realize because during such measurements, either cornea or lens show no specular reflection which is also detectable by the device due to the tilted position of the measuring beam with regard to the boundary layers. Thus, newer methods which promise increased reliability regarding the selection of the intraocular lenses and require the measuring of anterior chamber depth, lens thickness, or lens radii are not possible or only possible with difficulty.
According to a second method described in the literature [3], the measurement of intraocular distances takes place using one or more so-called B-scans which are obtained using optical coherence tomography. This allows for the resolution of the front face of the cornea and the retina as well as further tissue structures. For example, cornea thickness, anterior chamber depth, and/or lens thickness can be determined.
For example, the basic principle of the OCT method described in U.S. Pat. No. 5,321,501 A is based on white light interferometry and compares the travel time of a signal using an interferometer (most commonly a Michelson or Mach-Zehnder interferometer). The arm with known optical path length is used as object-external reference for the measurement arm. The interference of the signals from both arms yields a pattern which allows for the determination of the relative optical path length within an A-scan (single depth signal). In one-dimensional scanning grid methods, the beam is then guided transversally in one or two directions, allowing for the recording of a two-dimensional B-scan or a three-dimensional tomogram. This results in a sufficient number of signals even in the B-scan because with this method both specular reflections and scattering in the object are detected.
However, unlike the double-beam method, such methods do not ensure through the measurement principle itself that the axis length (axial length of the eye), which is important for calculating the intraocular lenses, is measured along the correct axis (visual axis). That is because a recording and a signal are possible even if the measuring beam does not impinge perpendicularly on the front face of the cornea and/or is not aligned along the visual axis. The measurement along the device axis results thus in an A-scan which, seen individually, shows no discernible defect, even if it is not measured along the visual axis due to lack of fixation. However, deducting the axis length from measuring along the device axis would generally result in incorrect, systematically shortened measurements because the A-scan, due to poor alignment of the measuring device to the visual axis, eye movement, and/or lack of fixation, measures laterally too far off the visual axis which, in a typically convex eye, results in a shortening of the distance cornea/retina.
In general, such B-scans pose the problem of the lateral attribution of the B-scans in terms of the eye. Due to the temporal duration of one or more B-scans, the eye is not always well fixated during the recording of the B-scans. If said eye movement is not taken into consideration, one B-scan and the intraocular distances, evaluable in such a B-scan, are laterally offset in terms of the eye and thus incorrectly attributed.
Therefore, it is not ensured that the A-scan along the device axis and/or the A-scan which runs within a B-scan along the device axis effectively measures the eye length. Moreover, even with exact alignment, only a few A-scans—only those along the device axis—can be used for calculating the axial length, and so the measured axial length is fraught with a relatively high statistical uncertainty.